Road-speed transducers, rotational speed sensors and no-load contacts are representative of the many sensors which are utilized in methods of the kind mentioned above. These sensors, as many other sensors, have in common that they emit a base signal when the variable which they measure takes on a base value with the base signal being identical with the signal corresponding to an inoperable sensor and this condition will now be considered.
If a road-speed transducer malfunctions, then it emits no pulses. The same applies when the transducer is operational; however, the vehicle for which it is intended to measure the road speed stands still. No pulses are then emitted from a rotational speed transducer when it is either defective or the engine for which the speed is to be measured is at standstill or is in the starting phase or shortly after cutoff. A no-load contact is either then closed when a driver presses against the actuating rod or when the driver does not press but the rod is hung up.
In conventional methods, a substitute or an emergency function is started as soon as it is detected that a sensor is not operational. For example, when the rotational speed transducer is inoperative, the rotational speed signal is derived from the signal of an injection-start transducer. This is then a substitute function. In contrast, if the road-speed transducer is inoperative, then the rotational speed of the engine is as a rule limited and this is then an emergency function.
When the variable being checked takes on its base value, it is then not possible for most sensors to reliably decide if it is operational or not. Therefore, substitute/emergency-functions are only started when a determination is made that the checked sensor operates defectively from a plausibility comparison of the signal from the checked sensor with signals from other sensors.
Efforts have been made to configure the above-mentioned plausibility checks so that it can be decided as rapidly as possible whether a sensor is still operational or not in order to trigger the substitute/emergency-functions as rapidly as possible.